1. Field of the Invention
The present invention relates to a semiconductor laser drive control circuit for an image forming system using the electrophotographic process, such as a laser printer or a digital copier. Further, the present invention relates to an image forming system which incorporates the semiconductor laser drive control circuit to control the laser beam emission of semiconductor lasers.
2. Description of the Related Art
FIG. 12 shows an essential part of an image forming system that uses an electrophotographic process in order to form an image on a photosensitive medium by using a laser beam emitted by a semiconductor laser.
In the image forming system of FIG. 12, a polygonal mirror 1 is rotated and a semiconductor laser unit 2 emits a laser beam to one of reflection surfaces of the rotating polygonal mirror 1. The semiconductor laser unit 2 is driven by a laser driver 7 in accordance with image data supplied to the laser driver 7.
In the image forming system of FIG. 12, the laser beam from the semiconductor laser unit 2 is deflected by one of the reflection surfaces of the rotating polygonal mirror 1. The deflected laser beam is passed through an fxcex8 lens 3, and the converging laser beam from the fxcex8 lens 3 hits the surface of a photosensitive medium 4.
As the polygonal mirror 1 is rotated around its rotation axis, the photosensitive medium 3 is scanned by the laser beam along the main scanning line. As the photosensitive medium 3 is rotated around its rotation axis, the photosensitive medium 3 is scanned by the laser beam along the sub-scanning line so that an electrostatic latent image is formed on the photosensitive medium 4 by using the laser beam emitted by the semiconductor laser unit 2. At the same time, a monitoring photodetector 9 receives the laser beam deflected by the polygonal mirror 1 and outputs a detection signal to a phase-locked loop (PLL) 6. A clock generator 8 outputs a clock signal to the PLL 6. The PLL 6 produces a line-sync clock signal, the phase of which is synchronized to the phase of the detection signal supplied by the monitoring photodetector 9, and the line-sync clock signal is sent back to an image processing unit 5, which serves as the source of the image data to form an image on the photosensitive medium 4.
The image forming system of FIG. 12 controls a time of the laser beam emission of the semiconductor laser 2 by using the laser driver 7, and the photosensitive medium 4 is exposed to the laser beam emitted at a controlled timing synchronized with the line-sync clock signal output by the phase-locked loop (PLL) 6. The image processing unit 5 supplies a clock signal synchronized with the line-sync clock signal from the PLL 6, to the laser driver 7, while supplying the data signal to the laser driver 7 in parallel to the clock signal. In this manner, the image forming system of FIG. 12 forms an electrostatic latent image on the photosensitive medium by using the laser beam.
As disclosed in Japanese Laid-Open Patent Applications No. 5-075199, No. 5-235446 and No. 9-321376, a semiconductor laser drive control circuit which controls the time of the laser beam emission of a semiconductor laser is known.
In the conventional semiconductor laser drive control circuit of the type disclosed in the above-mentioned publications, an optical-electrical load feedback loop is provided to amplify a differential current between an emission command signal and a detection signal by using a differential amplifier. The optical-electrical load feedback loop controls the forward current of the semiconductor laser. An automatic setting circuit is provided to set a conversion rate of the conversion of the emission command signal into a drive current of the semiconductor laser. A drive circuit drives the semiconductor laser so as to attain an optical level corresponding to the emission command signal independently of the optical-electrical load feedback loop. A controlled variable of the optical-electrical load feedback loop is decreased to improve high-speed modulation characteristics.
However, in order to attain the positional or dimensional accuracy of the electrostatic latent image formed on the photosensitive medium, it is absolutely necessary for the above-mentioned semiconductor laser drive control circuit to supply the clock signal to various elements of the above-mentioned control circuit, such as the image processing circuit 5 and the laser driver 7, as shown in FIG. 12. The above-mentioned control circuit requires various signal transmission lines provided therein, in order to supply the clock signal to the related circuit elements. Hence, the problem of electromagnetic interference (EMI) in the above-mentioned control circuit will arise. At the same time, the cost of the above-mentioned control circuit will be raised because of the various signal transmission lines and the related circuit elements.
Further, with a recent demand for high-speed, high-density image formation in image forming systems, an improved image forming system, which is provided with two or more semiconductor lasers as the laser light source, has been proposed. There are two schemes of the improved image forming system. One scheme is the use of a semiconductor laser array containing two or more semiconductor lasers for the improved image forming system. The other is the use of two or more separate semiconductor lasers for the improved image forming system.
In the laser-array type image forming system, the monitoring photodetector is shared by the individual semiconductor lasers contained in the semiconductor laser array, and the conventional semiconductor laser drive control method of the above-mentioned publications cannot suitably be applied to the laser-array type with low cost. If additional circuit elements and additional signal transmission lines are used to incorporate the conventional semiconductor laser drive control method into the laser-array type, the cost will be unnecessarily increased.
Accordingly, the latter scheme in which two or more separate semiconductor lasers are used for the improved image forming system has an advantage over the former scheme.
It is conceivable that the conventional semiconductor laser drive control method of the above-mentioned publications be applied to the separate-laser type image forming system. However, in the separate-laser type image forming system, when the optical level output by each semiconductor laser received at the monitoring photodetector is low, the linearity of the conversion of the optical level into the electrical drive current by the feedback loop will be significantly degraded. In some case in which the monitored optical level is low, the drive current of the semiconductor laser becomes too high, which causes a defect in the background part of the image reproduced by the image forming system.
Further, in the separate-laser type image forming system, the optical-electrical load feedback loop always controls the drive current of each semiconductor laser. It is difficult to reset the drive current of the semiconductor laser to zero, so as to completely turn off the semiconductor laser. If an additional control element which forcefully resets the drive current of the semiconductor laser to zero is provided in the image forming system, the cost will be increased further.
In order to overcome the problems described above, preferred embodiments of the present invention provide an improved semiconductor laser drive control circuit that provides a simple, inexpensive configuration and carries out accurate control of the laser beam emission of each of a plurality of semiconductor lasers provided in an image forming system to attain a high-speed, high-density image formation.
Another object of the present invention is to provide a laser drive controller that provides a simple, inexpensive configuration and carries out accurate control of the laser beam emission of a semiconductor laser of an image forming system to attain a high-speed, high-density image formation.
Another object of the present invention is to provide an improved image forming system that incorporates the semiconductor laser drive control circuit, together with the plurality of semiconductor lasers, the image forming system providing a simple, inexpensive configuration and carrying out accurate control of the time of the laser beam emission of the plurality of semiconductor lasers.
According to one preferred embodiment of the present invention, a laser drive controller includes a first hold/output unit which holds a maximum emission voltage of a semiconductor laser and supplies the emission voltage, held by the first hold/output unit, to the semiconductor laser when an emission command signal is set; and a second hold/output unit which sets a bias current in the semiconductor laser and supplies a bias voltage, held by the second hold/output unit, to the semiconductor laser when the emission command signal is reset.
According to another preferred embodiment of the present invention, a semiconductor laser drive control circuit for an image forming system including N semiconductor lasers provided therein, where N is an integer larger than or equal to 2, includes: a PLL circuit which includes a voltage-controlled oscillator, a programmable counter and a phase detector, the phase detector detecting an error between a phase of a load signal output by the programmable counter and a phase of a reference-frequency signal and outputting a phase-error signal indicative of the detected error, the programmable counter determining a frequency of an output clock signal of the oscillator that is equal to a frequency of an input clock signal of the oscillator divided by a division factor; N pixel clock output units each of which produces a corresponding pixel clock signal for one of the N semiconductor lasers from the output clock signal of the oscillator; and N laser drive controllers each of which produces a corresponding drive signal for one of the N semiconductor lasers from the pixel clock signal produced by a corresponding one of the N pixel clock output units, the drive signal being used to control a laser beam emission of the one of the N semiconductor lasers, each laser drive controller including: a first hold/output unit which holds a maximum emission voltage of one of the N semiconductor lasers, and supplies the emission voltage, held by the first hold/output unit, to the one of the N semiconductor lasers when an emission command signal is set; and a second hold/output unit which sets a bias current in said one of the N semiconductor lasers, and supplies a bias voltage, held by the second hold/output unit, to the one of the N semiconductor lasers when the emission command signal is reset.
According to another preferred embodiment of the present invention, an image forming system includes N semiconductor lasers where N is an integer larger than or equal to 2, and a semiconductor laser drive control circuit which controls a laser beam emission of each of the N semiconductor lasers, the semiconductor laser drive control circuit including: a PLL circuit which includes a voltage-controlled oscillator, a programmable counter and a phase detector, the phase detector detecting an error between a phase of a load signal output by the programmable counter and a phase of a reference-frequency signal and outputting a phase-error signal indicative of the detected error, the programmable counter determining a frequency of an output clock signal of the oscillator that is equal to a frequency of an input clock signal of the oscillator divided by a division factor; N pixel clock output units each of which produces a corresponding pixel clock signal for one of the N semiconductor lasers from the output clock signal of the oscillator; and N laser drive controllers each of which produces a corresponding drive signal for one of the N semiconductor lasers from the pixel clock signal produced by a corresponding one of the N pixel clock output units, the drive signal being used to control a laser beam emission of the one of the N semiconductor lasers, each laser drive controller including: a first hold/output unit which holds a maximum emission voltage of one of the N semiconductor lasers, and supplies the emission voltage, held by the first hold/output unit, to the one of the N semiconductor lasers when an emission command signal is set; and a second hold/output unit which sets a bias current in the one of the N semiconductor lasers, and supplies a bias voltage, held by the second hold/output unit, to the one of the N semiconductor lasers when the emission command signal is reset.
The semiconductor laser drive control circuit of the present invention is effective in providing a simple, inexpensive configuration and in carrying out accurate control of the laser beam emission of each of the semiconductor lasers.
The semiconductor laser drive control circuit of the present invention can provide a fine, accurate oscillation frequency of the clock signal at the output of the voltage-controlled oscillator in the PLL circuit by setting the division factor of the programmable counter to an appropriate number. Hence, it is possible for the semiconductor laser drive control circuit to prevent the degradation of the linearity of the optical-to-electrical conversion by the feedback loop, as in the conventional control circuit, when the optical output level of the semiconductor laser is low.